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Contracts and Agreements (Additional Information) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Long-Duration Contracts, Assumptions by Product and Guarantee [Line Items]
Annual Fee for transfer agency services	$ 20,000us-gaap_NoninterestExpenseTransferAgentAndCustodianFees
Ratio of expense to Average Net Assets	0.01%usl_RatioOfExpensesToAverageNetAssets	0.01%usl_RatioOfExpensesToAverageNetAssets	0.02%usl_RatioOfExpensesToAverageNetAssets
Maximum
Long-Duration Contracts, Assumptions by Product and Guarantee [Line Items]
Annual Fee for transfer agency services	75,000us-gaap_NoninterestExpenseTransferAgentAndCustodianFees / us-gaap_RangeAxis = us-gaap_MaximumMember
Transaction fees per transaction	15usl_TransactionFeesPaidPerTransaction / us-gaap_RangeAxis = us-gaap_MaximumMember
Commissions per round-turn trade, including applicable exchange and NFA fees for Oil Futures Contracts and options on Oil Futures Contracts	15usl_CommissionsPaid / us-gaap_RangeAxis = us-gaap_MaximumMember
Minimum
Long-Duration Contracts, Assumptions by Product and Guarantee [Line Items]
Annual Fee for transfer agency services	75,000us-gaap_NoninterestExpenseTransferAgentAndCustodianFees / us-gaap_RangeAxis = us-gaap_MinimumMember
Transaction fees per transaction	7usl_TransactionFeesPaidPerTransaction / us-gaap_RangeAxis = us-gaap_MinimumMember
Commissions per round-turn trade, including applicable exchange and NFA fees for Oil Futures Contracts and options on Oil Futures Contracts	7usl_CommissionsPaid / us-gaap_RangeAxis = us-gaap_MinimumMember
First $500 million of USL's, USOF's, USNG's, UGA's, USDHO's, USSO's, US12NG's, USBO's, USCI's, CPER's, USAG's and USMI's combined net assets
Long-Duration Contracts, Assumptions by Product and Guarantee [Line Items]
Fee percentage	0.06%usl_BaseFeePercentage / invest_InvestmentIdentifierSchemeAxis = usl_GroupSixMember
First $500 million of USL's, USOF's, USNG's, UGA's, USDHO's, USSO's, US12NG's, USBO's, USCI's, CPER's, USAG's and USMI's combined net assets | Maximum
Long-Duration Contracts, Assumptions by Product and Guarantee [Line Items]
Base amount for determining fee percentage	500,000,000usl_BasisAmountForFeePercentage / invest_InvestmentIdentifierSchemeAxis = usl_GroupSixMember / us-gaap_RangeAxis = us-gaap_MaximumMember
USL's, USOF's, USNG's, UGA's, USDHO's, USSO's, US12NG's, USBO's, USCI's, CPER's, USAG's and USMI's combined net assets greater than $500 million but less than $1 billion
Long-Duration Contracts, Assumptions by Product and Guarantee [Line Items]
Fee percentage	0.0465%usl_BaseFeePercentage / invest_InvestmentIdentifierSchemeAxis = usl_GroupSevenMember
USL's, USOF's, USNG's, UGA's, USDHO's, USSO's, US12NG's, USBO's, USCI's, CPER's, USAG's and USMI's combined net assets greater than $500 million but less than $1 billion | Maximum
Long-Duration Contracts, Assumptions by Product and Guarantee [Line Items]
Base amount for determining fee percentage	1,000,000,000usl_BasisAmountForFeePercentage / invest_InvestmentIdentifierSchemeAxis = usl_GroupSevenMember / us-gaap_RangeAxis = us-gaap_MaximumMember
USL's, USOF's, USNG's, UGA's, USDHO's, USSO's, US12NG's, USBO's, USCI's, CPER's, USAG's and USMI's combined net assets greater than $500 million but less than $1 billion | Minimum
Long-Duration Contracts, Assumptions by Product and Guarantee [Line Items]
Base amount for determining fee percentage	500,000,000usl_BasisAmountForFeePercentage / invest_InvestmentIdentifierSchemeAxis = usl_GroupSevenMember / us-gaap_RangeAxis = us-gaap_MinimumMember
USL's, USOF's, USNG's, UGA's, USDHO's, USSO's, US12NG's, USBO's, USCI's, CPER's, USAG's and USMI's combined net assets exceed $1 billion
Long-Duration Contracts, Assumptions by Product and Guarantee [Line Items]
Fee percentage	0.035%usl_BaseFeePercentage / invest_InvestmentIdentifierSchemeAxis = usl_GroupEightMember
USL's, USOF's, USNG's, UGA's, USDHO's, USSO's, US12NG's, USBO's, USCI's, CPER's, USAG's and USMI's combined net assets exceed $1 billion | Minimum
Long-Duration Contracts, Assumptions by Product and Guarantee [Line Items]
Base amount for determining fee percentage	1,000,000,000usl_BasisAmountForFeePercentage / invest_InvestmentIdentifierSchemeAxis = usl_GroupEightMember / us-gaap_RangeAxis = us-gaap_MinimumMember
Marketing Agreement | Maximum
Long-Duration Contracts, Assumptions by Product and Guarantee [Line Items]
Fee percentage	10.00%usl_BaseFeePercentage / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_TypeOfArrangementAxis = usl_MarketingAgreementMember
Marketing Agreement | USL's assets up to $3 billion
Long-Duration Contracts, Assumptions by Product and Guarantee [Line Items]
Fee percentage	0.06%usl_BaseFeePercentage / invest_InvestmentIdentifierSchemeAxis = usl_GroupFourMember / us-gaap_TypeOfArrangementAxis = usl_MarketingAgreementMember
Marketing Agreement | USL's assets up to $3 billion | Maximum
Long-Duration Contracts, Assumptions by Product and Guarantee [Line Items]
Base amount for determining fee percentage	3,000,000,000usl_BasisAmountForFeePercentage / invest_InvestmentIdentifierSchemeAxis = usl_GroupFourMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_TypeOfArrangementAxis = usl_MarketingAgreementMember
Marketing Agreement | USL's assets in excess of $3 billion
Long-Duration Contracts, Assumptions by Product and Guarantee [Line Items]
Fee percentage	0.04%usl_BaseFeePercentage / invest_InvestmentIdentifierSchemeAxis = usl_GroupFiveMember / us-gaap_TypeOfArrangementAxis = usl_MarketingAgreementMember
Marketing Agreement | USL's assets in excess of $3 billion | Minimum
Long-Duration Contracts, Assumptions by Product and Guarantee [Line Items]
Base amount for determining fee percentage	$ 3,000,000,000usl_BasisAmountForFeePercentage / invest_InvestmentIdentifierSchemeAxis = usl_GroupFiveMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_TypeOfArrangementAxis = usl_MarketingAgreementMember

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- Definition

Fees paid to an agent employed by a corporation or mutual fund to maintain shareholder records, including purchases, sales, and account balances. Also includes custodian fees incurred during an accounting period from an agent, bank, trust company, or other organization that holds and safeguards an individual's, mutual fund's, or investment company's assets for them. These fees will be billed back to the client and are a component of noninterest expense.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-04.14)

-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

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Commissions Paid

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- Definition

Ratio Of Expenses To Average Net Assets

+ References

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Transaction Fees Paid.

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